Nitrous oxide (N2O) is a greenhouse gas with a global warming potential of 310 times that of CO2 and an atmospheric lifetime of 114 years. Automotive exhaust is one source of N2O emissions, as a by-product of fuel combustion and as a by-product formed during the catalytic reduction of nitrogen oxides (NOx). N2O is formed under transient conditions over all major classes of emission control catalysts, including three-way conversion (TWC) catalysts for traditional/stoichiometric gasoline cars and for gasoline direct injection (GDI) gasoline cars, diesel oxidation catalysts (DOCs), catalyzed soot filters (CSFs), lean NOx traps (LNTs), selective catalytic reduction catalysts (SCRs), which reduce NO, with urea, and selective ammonia oxidation catalysts (AMOx) catalysts for diesel vehicles.
Recognizing its global warming potential, US EPA has already set a N2O emission limit of 10 mg/mile for light-duty vehicles over the FTP cycle starting from MY2012, and a N2O emission limit of 0.1 g/bhp-h for heavy duty vehicles over the heavy duty FTP cycle starting from MY2014. In the past, automobile catalyst systems were optimized for maximum reduction of NOx (a regulated pollutant) without accounting for N2O level. The more stringent regulations currently on N2O emissions require that the emission control system design be optimized not only for high NOx conversion performance but also for low N2O emissions. Under the present standards, if N2O exceeds the 10 mg/mile limits, there is a penalty against CAFE fuel economy requirements.
It is generally understood that N2O can be decomposed industrially, e.g., in the context of treating off-gases from nitric acid and adipic acid production. The temperatures for these operations are much higher (>550° C., for example, about 800-900° C.) than the temperature of typical automotive exhaust, and the process streams for these operations contain little water (<1%), unlike typical exhaust gas streams. There are many literature reports describing N2O decomposition catalysts, and most can be grouped into three categories: (1) supported rhodium (Rh), (2) metal oxides with a spinel structure and (3) ion-exchanged zeolites. Such catalysts are usually in powder or pelleted form. In DE102008048159, decomposition of N2O in a gas stream is conducted with a catalyst comprising rhodium supported on a gamma-alumina that is optionally doped with cerium (Ce) or gold (Au).
KR20060019035 is directed to a method for removing nitrogen oxides using dual catalyst beds, wherein nitrogen oxides are decomposed into nitrogen and nitrous oxide using a bed of nitrogen oxide-reducing catalyst Pt/VX-PY-(material containing hydroxyl group)z, and the nitrous oxide thus formed is then further decomposed into nitrogen and oxide using a bed of a nitrous oxide-decomposing catalyst comprising Rh and silver (Ag), namely, Rh-Ag/CeO2/M1-M2-M3, where M1 is magnesium (Mg), barium (Ba) or strontium (Sr), M2 is aluminum (Al), iron (Fe), vanadium (V), gallium (Ga) or chromium (Cr), and M3 is zinc (Zn), nickel (Ni), or copper (Cu). There is a continuing need in the art to provide catalytic articles that efficiently and effectively provide removal of nitrous oxide (N2O), particularly under exhaust gas conditions.